1. Field of the Invention
The invention is related to an actuator for a drum brake, comprising a housing, a screw mechanism comprising at least one screw and a nut, which screw mechanism is supported with respect to the housing, and drive means which are drivingly connected to the screw mechanism for providing a linear movement of at least one of the screw and nut with respect to the housing upon driving the screw the mechanism.
2. Description of Related Art
Such actuator is known from U.S. Pat. No. 5,310,026 to Shaw et al. It comprises a screw which is connected to the actuator housing by means of a thrust bearing. The associated nut is held slidably within the housing, and is driven linearly by rotating the screw. Upon actuation, a linear force is exerted through the nut and the housing. This housing should therefore be movable, so as to provide e.g., a balanced braking action in a drum brake.
Such actuator is known from U.S. Pat. No. 5,310,026. It comprises a screw which is connected to the actuator housing by means of a thrust bearing. The associated nut is held slidably within the housing, and is driven linearly by rotating the screw. Upon actuation, a linear force is exerted through the nut and the housing. This housing should therefore be movable, so as to provide e.g. a balanced braking action in a drum brake.
The object of the invention is to provide an improved actuator, in which the housing is not part of the force trajectory. This object is achieved in that the screw mechanism comprises two axially aligned screws which have opposite lead angles and two corresponding nuts, said screws each engaging a respective nut, the drive means being connected to both screws or to both nuts.
The actuator according to the invention provides a driving action through the pair of nuts or screws, without exerting loads on the housing. As a result, the housing can be carried out as a relatively simple component, e.g., a cylindrical tube, the nuts having a corresponding cylindrical circumference and fitting slidably within the cylindrical tube.
At their ends, the nuts may have an actuating member which can take several forms, dependant on the envisaged application of the actuator.
At or near the ends of the nuts facing away from each other a seal, e.g., a sealing bellows is provided, which seals are connected to the respective ends of the cylindrical tube. The internally located screw thread of nuts and screws is thereby well protected against dirt, etcetera. This is particularly of importance in the case of ball or roller screws.
The drive means are drivable through an opening in the housing, and can be carried out as a splinexe2x80x94, e.g., a screw gear wheel. Furthermore, the drive means may comprise a motor, the stator of which engages the housing, and the rotor of which is drivingly connected to the spline.
According to a preferred embodiment, the motor is drivably connected to a drive gear wheel which meshes with the spline, said drive gear wheel being of greater width than the spline so as to allow axial displacements of the screws and spline while maintaining the mesh.
As a result, the nuts which are driven towards or away from each other can adapt themselves freely to the surrounding structure through the actuating heads thereof.
The maximum diameter of the spline is smaller than the diameter of the bore of the cylindrical tube, the nuts at their side facing the spline being recessed so as to allow displacement of said nuts along said drive gear wheel.
The housing and the nuts can be held non-rotatably with respect to each other through a groove-pin connection.
The invention is furthermore related to an arrangement for for a drum brake, comprising a backing plate to be connected to a vehicle wheel suspension, e.g., a steering knuckle, housing etc., a pair of brake shoes supported on said backing plate, a screw actuator engaging said brake shoes and a motor for driving said screw actuator so as to obtain a linear movement thereof for displacing the brake shoes outwardly and away from each other vice versa, said actuator comprising a housing and a screw mechanism which is supported with respect to the housing, and with at least one screw and a nut.
Such arrangement is known from U.S. Pat. No. 5,310,026 as well.
According to the invention, the screw mechanism comprises two axially aligned screws which have opposite lead angles and two corresponding nuts, said screws each engaging a respective nut the drive means being connected to both screws or to both nuts.
The actuating members may, according to a first possibility, be connected pivotably to a respective brake shoe. Alternatively, each actuating member may have an abutment surface, the brake shoes being held under pretension against a respective abutment surface by means of a spring.
In the latter case, each nut is held non-rotatably in the cylindrical tube. In case of a pivotable connection between actuating members and brake shoes, no further precautions are necessary for preventing mutual rotations of the nuts with respect to the housing.
The housing can be fixedly connected to the backing plate.
The drive means may comprise a magnet coupling for holding the braking torque under currentless condition (hand brake).